The present invention relates to an electrophotographic photoreceptor, more specifically to an electrophotographic photoreceptor using a photoconductive material comprised of mixed crystals of a titanylphthalocyanine and a vanadylphthalocyanine, useful for the use of printers and copying machines, and suitable for the image formation by use of semi-conductor laser beams or LED beams as an exposing means.
In recent years, photoconductive materials are intensively studied and used as a photoelectric transfer element in electrophotographic photoreceptors, solar batteries and image sensors. As such photoconductive materials, inorganic materials have been widely used so far. In electrophotographic photoreceptors, for example, there have been mostly used inorganic photoreceptors having a photoreceptive layer whose primary component is an inorganic photoconductive material such as selenium, zinc oxide or cadmium sulfide.
However, such inorganic photoreceptors are not necessarily satisfactory in the properties of photosensitivity, heat stability, moisture resistance and durability, which are required of electrophotographic photoreceptors for copying machines and printers. Selenium, for example, is liable to crystallize with heat or stains such as fingerprints and thereby deteriorates in properties required of electrophotographic photoreceptors. An electrophotographic photoreceptor using cadmium sulfide is poor in moisture resistance and durability, and that using zinc oxide has a problem in durability.
Further, in the growing importance of improving environmental sanitation, electrophotographic photoreceptors comprised of selenium or cadmium sulfide have a disadvantage of requiring a rigid control in manufacturing and handling because of their toxicity.
In order to eliminate these shortcomings of inorganic photoconductive materials, organic photoconductive materials have come to be actively studied, and various attempts have been made concerning the use of them in a photoreceptive layer of electrophotographic photoreceptor. Japanese Pat. Exam. Pub. No. 10496/1975 discloses an organic photoreceptor having a photoreceptive layer containing poly-N-vinylcarbazole and trinitrofluorenone, but this photoreceptor is not adequate in sensitivity and durability. For the purpose of solving these problems, a function-separating electrophotographic photoreceptor has been developed, in which a carrier generation function and a carrier transfer function are separately provided by different materials.
Such function-separating photoreceptors have an advantage that materials having desired characteristics can be selected from a wide range of compounds to prepare with ease photoreceptors of high sensitivity and excellent durability.
Various organic compounds have been proposed as a carrier generation material or a carrier transfer material for electrophotographic photoreceptors. As the carrier generation material which controls the basic characteristics of a photoreceptor, there have come to be practically used photoconductive materials such as polycyclic quinones represented by dibromoanthanthrone, pyrylium compounds and their eutectic complexes, squarium compounds, phthalocyanine compounds and azo compounds.
However, carrier generation materials having a much higher carrier generation efficiency are required to improve the sensitivity of organic photoreceptors much more. From this viewpoint, phthalocyanine compounds have come to draw much attention for their high photoconductivity, and active studied are being made in connection with their application.
It is known that phthalocyanines vary in physical properties such as absorption spectrum and photoconductivity according to their crystal forms and the kind of the central metal. For example, M. Sawata reports, in Senryo To Yakuhin, 24 (6), 122 (1979), that copper phthalocyanine has four crystal forms of type-, -.alpha., -.beta., -.gamma. and -.epsilon. which are greatly different in electrophotographic properties.
In addition, four primary crystal forms of type-A, -B, -C and -Y are also reported for titanylphthalocyanines which attract much attention in recent years . However, any of type-A titanylphthalocyanine disclosed in Japanese Pat. O.P.I. Pub. No. 67094/1987, type-B disclosed in Japanese Pat. O.P.I. Pub. No. 239248/1986 and type-C disclosed in Japanese Pat . O.P.I. Pub. No. 256865/1987 is not necessarily satisfactory in electrification property and electrophotographic sensitivity. Titanylphthalocyanine reported recently by Oda et al. in Electrophotography, 29 (3), 250 (1990) has a high sensitivity, but it is not satisfactory in electrification property; therefore, development of a carrier generation material high in both electrification property and sensitivity is demanded.
Vanadylphthalocyanines also appear in research reports and patents frequently. Japanese Pat. O.P.I. Pub. No. 217074/1989 discloses a photoreceptor containing a vanadylphthalocyanine of which crystal form is corresponding to the crystal form of type-B titanylphthalocyanine, and Japanese Pat. O.P.I. Pub. No. 204968/1989 discloses one comprised of vanadylphthalocyanine having a crystal form corresponding to that of type-A, but these crystal forms cannot provide an adequate sensitivity. In addition, Japanese Pat. O.P.I. Pub. No. 268763/1989 discloses use of the crystal form which has a characteristic peak at a Bragg angle (2.theta.) of 27.2.degree., like the crystal form of titanylphthalocyanine shown as a comparative example in Japanese Pat. O.P.I. Pub. No. 67094/1987. But its sensitivity is not adequate, either. The reason of this lies in the fact that the crystal forms of both the vanadylphthalocyanine and the titanylphthalocyanine having a characteristic peak only at a Bragg angle (2.theta.) of 27.2.degree. are distinctly different in three-dimensional crystal configuration from the crystal form of high sensitive type-Y titanylphthalocyanine, which has another characteristic peak at 9.5.degree.. As described above, there has not been reported so far a crystal form of vanadylphthalocyanine which can provide a high sensitivity.
Recently, mixed crystals of phthalocyanines are reported, in which a specific crystal configuration is formed by use of plural phthalocyanines. These mixed crystals are greatly different from a mere mixture of plural phthalocyanines and have an advantage of providing properties different from those of a single phthalocyanine or a mixture thereof. In connection with such mixed crystals, Japanese Pat. O.P.I. Pub. No. 84661/1990 discloses the formation of mixed crystals on a substrate by co-deposition of two or more phthalocyanines from a gas phase. But the crystal form of mixed crystals between a copper phthalocyanine and a nonmetal phthalocyanine, as well as that of mixed crystals between a titanylphthalocyanine and a nonmetal phthalocyanine, each described therein have problems in sensitivity. Japanese Pat. O.P.I. Pub. No. 70763/1990 discloses two types of mixed crystals prepared by vapor deposition of a titanylphthalocyanine and a vanadylphthalocyanine, which correspond to type-A and type-B titanylphthalocyanines, respectively, but their sensitivities are unsatisfactory. As described above, it is important to select properly the crystal form of mixed crystals and the types of phthalocyanines used to form mixed crystals, otherwise mixed crystals of desired properties cannot be obtained. From this point of view, not only the selection of the materials but also a crystal-controlling technique to obtain a specific crystal form are important; therefore, development of a crystal conversion technique is demanded, in addition to the conventional vapor deposition method to form mixed crystals.